Canon CXDI-1 Operation Manual page 83

REX
When REX is adjusted, the LUT graph shifts
in the pixel value direction (right figure).
An enlarged graph is shown below.
Extracted analysis
value position
Analysis value = 1220
Density = 1.9 D
(Second quadrant)
X-Ray dose
(Third quadrant)
Sensor Characteristics Curve
This graph depicts an REX for a chest front exposure.
The name of the quadrants, from the upper right in counterclockwise direction, are as
follows: the first quadrant, the second quadrant, the third quadrant and the fourth
quadrant. The first quadrant is the gray scale curve showing the relationship between
pixel and density. The third quadrant is the sensor characteristics curve, showing the
relationship between the X-ray dose that has reached the sensor and the sensor output
value. The fourth quadrant is the logarithm conversion curve, which converts the sensor
output value (proportional to the X-ray dose) into a pixel value proportional to the
logarithm of X-ray dose. Only the right half of the graph (the first and the fourth
quadrants) will be explained here to simplify the explanation.
First, an explanation of the algorithm that determines the gray scale curve
The CXDI selects the maximum pixel value of the upper lung area as the analysis value
for chest PA exposure. In the above graph, the analysis value is 1220. Suppose 1.9 D has
been specified as the density at the analysis value position. The gray scale curve shifts in
the horizontal direction. The amount by which it shifts is determined by the position
where the arrow from the analysis value intersects with the arrow from the density.
Next, an explanation of the algorithm that determines the REX.
The REX is determined by using the gray scale curve. The position where the arrow
from the gray scale curve at reference density 0.75 D (fixed) intersects with the
logarithm conversion curve is the REX. Thus, the sensor output value corresponding to
the area expressed as density 0.75 D is the REX.
0.75 D is the density when the printer LUT is linear, the maximum density is 3.2 D, and
the minimum density is 0.2 D.
7. Processing the Image (QA Mode Screen)
Density
Density
Low X-ray dose
(High sensitivity)
Density = 1.9 D
Reference
density = 0.75 D
REX = 375
Analysis value = 1220
Sensor output
(14 bit linear)
– 75 –
Pixel
DOWN UP
Value
REX
Gray scale curve
High X-ray dose
(Low sensitivity)
(First quadrant)
Pixel value
(12 bit log)
(Fourth quadrant)
Logarithm conversion curve